Method of detecting a malfunction during a displacement of an element by means of a drive system, and device suitable for carrying out such a method

ABSTRACT

A method and a device are provided for detecting the occurrence of a malfunction upon movement of an element by a driving system. While the element is being moved, a difference between a predetermined value and an actual value is determined at regular intervals by means of a processor. A derivative of the difference is determined by the processor at regular intervals. The difference and the derivative both fluctuate around an equilibrium value. Subsequently, only the values on one side of the equilibrium value of both the difference and the derivative are sampled. The sampled values are multiplied and the result is compared with a reference value by means of the processor. The occurrence of a malfunction is established if the result of the multiplication is higher than the reference value.

The invention relates to a method of detecting a malfunction during adisplacement of an element by a drive system, in which method aprocessor determines a difference between a predetermined desired valueand an actual value at regular intervals during the displacement of theelement.

The invention further relates to a device suitable for carrying out sucha method.

Such a method is known per se from EP-B1-0 365 681. A collision betweenmachine parts driven by servomotors and an object can be detected bymeans of the method described therein. A processor calculates thederivative of the speed of the servomotor during a preceding period andsubtracts it from the derivative of the speed of the servomotor over thepresent period. The absolute value of the calculated difference istaken, and this is compared by the processor with a given referencevalue. If the value is greater than the given reference value, this isinterpreted as an indication that there is a collision.

Such a method has the disadvantage that the detection of a collisiontakes comparatively much time because the minimum time required fordetecting the collision is equal to the length of the chosen timeperiod, with the result that the malfunction may already have occurredat the beginning of the measured period.

The invention has for its object to provide a method wherein the timerequired for detecting a malfunction is comparatively short.

This object is achieved in the method according to the invention in thatthe processor further determines a derivative of the difference atregular intervals, said difference and its derivative both fluctuatingaround an equilibrium value, whereupon only the values at one side ofthe equilibrium value of the both the difference and the derivative aretaken, the values of the difference are multiplied by the value of thederivative, the outcome of the multiplication is compared with areference value by the processor, and a malfunction in the displacementof the element is detected if the outcome of the multiplication ishigher than the reference value.

The multiplication of the value of the derivative of the difference bythe value of the difference at a single side of the equilibrium valuegenerates a curve in time which has a comparatively steeper slope (alsodenoted comparatively great directional coefficient) in the case of amalfunction than the curve of the value of the derivative and/or thecurve of the difference value. As a result, the curve will risecomparatively quickly in the case of a malfunction, so that thereference value is reached comparatively quickly, and a malfunction canbe ascertained and detected.

An embodiment of the method according to the invention is characterizedin that the chosen side of the equilibrium value is dependent on thedirection in which the element is displaced.

If the element has a certain speed in a certain direction, the speed insaid direction will drop below the desired value the moment a collisionoccurs. This information is relevant for detecting a collision. Theinformation that the speed of the element is higher than the desiredvalue is of no importance in such a case and may accordingly be set forzero.

Another embodiment of the method according to the invention ischaracterized in that the signals of the derivative are filtered.

An advantage of this is that exclusively those signals remain owing tothe signal filtering which are relevant for making a malfunctiondetection possible.

A further embodiment of the method according to the invention ischaracterized in that the predetermined desired value represents thedesired position of the displaceable element, while the actual valuerepresents the actual position of the element.

An advantage of such a method is that the element can be accuratelydisplaced into a desired position, while a malfunction during thedisplacement, such as a collision, is detected comparatively quickly.

It is a further object of the invention to provide a device by means ofwhich a malfunction in a drive system for the displacement of an elementcan be detected comparatively quickly.

This object is achieved in the device according to the invention in thatthe device is provided with an element that is displaceable by means ofa drive system and with a processor provided with means for comparing adesired value with an actual value, means for determining a derivative,means for determining values lying at one side of an equilibrium value,multiplication means, and means for comparing the outcome of amultiplication with a reference value.

A malfunction, such as a collision, can be detected comparativelyquickly by means of such a device, as was described further above.

The invention will be explained in more detail below with reference tothe accompanying drawings, in which:

FIG. 1 shows a component placement device;

FIG. 2 shows a control circuit of the device shown in FIG. 1, in which afeedback between the processor and the drive system is shown;

FIG. 3 is a graph representing a difference between a desired value andan actual value in time, and a derivative thereof;

FIG. 4 is a graph showing curves representing the difference and thederivative of the difference after the positive part has been set forzero; and

FIG. 5 shows the graph of FIG. 4, now containing a further curvecorresponding to the multiplication of the curves shown in FIG. 4.

Corresponding components have been given the same reference numerals inthe Figures.

FIG. 1 shows a component placement device 1 which is provided with aframe 2. Rails 3 are situated on the frame 2 at both sides. A guide 4extends transversely to the rails 3 and is displaceable over the rails 3by means of a drive system (not shown) in and opposite to the directionindicated by arrow P1. An arm 5 is provided on the guide 4, which arm isdisplaceable by means of a drive system over the guide 4 in and oppositeto the direction indicated by arrow P2. The direction of arrow P2 isperpendicular to the direction of arrow P1. An imaging device 6 and aplacement device 7 are fastened to the arm 5.

A transport device 8, by means of which substrates 9 are displaceable inthe direction indicated by arrow P2, is present below the arm 5. Eachsubstrate 9 is provided with at least one reference element 10. Thecomponent placement machine 1 is further provided with a componentfeeder device 11 from which components can be taken by the placementdevice 7. A further imaging device 12 is located on the frame 2.

FIG. 2 shows a control circuit 13 of a processor of a componentplacement device 1, diagrammatically showing a controller 14 by means ofwhich a drive system 15 of the arm is controlled. The arm 5 will alwaysbe controlled such that the placement device 7 is displaced into adesired position. This desired position is applied to a differencedeterminator 16 via input element 17. The actual position of theplacement device 7 realized by the controller 14 and the drive system 15will also be applied to this difference determinator 16.

The control circuit 16 discussed thus far is known per se and willaccordingly not be described in any more detail.

FIG. 3 shows a graph with two curves, curve A representing thedifference between the desired position and the actual position of theplacement device 7 in time t, while curve B is the derivative of curve Ain time. As is apparent from the graph, the curves A and B vary aroundan equilibrium value 0. It is visible in the graph that a malfunctionoccurs in the curve A at a moment t_(v), such that the differencebetween the desired position and the actual position assumes aconsiderable negative value. Such a situation may arise, for example, ifthe placement device 7 is displaced in the direction indicated by arrowP1, which direction indicated by arrow P1 corresponds to the negativeX-direction. The curve B, being the derivative of curve A, first shows asteep drop, whereupon it assumes a constant negative value.

The malfunction may be caused, for example, by the fact that theplacement device 7 during its displacement hits against a componentalready provided on the substrate 9, with a collision as a result.

The actual position of the placement device 7 will never be beyond thedesired position in the direction indicated by arrow P1 in the case of acollision during the displacement of the placement device 7 in thedirection of arrow P1. This means that the part of the curve for whichthe difference between the desired position and the actual position ispositive may be disregarded. Accordingly, this value is set for zero inthe controller 14 in accordance with the method according to theinvention.

FIG. 4 shows the curves A and B of FIG. 3, with the parts of the curvesA and B situated above the equilibrium value 0 being reset to 0.

FIG. 5 shows the curves A and B of FIG. 4 as well as a curve C. Thecurve C is the multiplication of the curves A and B. The curve Coccasionally assumes positive values with a maximum amplitude R which isconsiderably smaller than the amplitudes of the curves A and B over thetime period from t₀ to t_(v). This amplitude over the indicated timeperiod t₀-t_(v) may serve as a reference value R for detecting a normaldisplacement.

From the moment t_(v) onwards, i.e. the moment the collision takesplace, the curve C rises with a very steep gradient. As is visible inFIG. 5, the accompanying value of curve C will be considerably higherthan the amplitude of curve C in the time period t₀-t_(v) a very shorttime after t_(v) already. The moment the reference value R is exceeded,which is the case comparatively soon after moment t_(v) already, asdescribed above, this can and will be regarded as a signal that acollision has taken place. The arm 5 should now be stopped by means ofthe processor or be displaced in a direction opposed to that of arrow P1so as to avoid damage to the placement device 7 and/or the substrate 9.

It is also possible to determine a higher-order derivative instead ofthe derivative of curve B. A higher-order derivative is more accurate,but also increases the required calculation time. Depending on thedesired application, a compromise between accuracy and the desiredcalculation time will have to be made.

The controller 14 may carry out a filtering function during determiningof the derivative, if so desired, so as to remove noise and otherundesirable effects from the measured curve.

It is also possible to supply a desired and actual speed, force, ortemperature to the difference determinator 16 instead of the desired andactual positions.

It will be obvious that the desired and actual positions of theplacement device 7 processed in the control circuit 13 may relate to theX- as well as to the Y- and Z-directions.

It is also possible to apply the method according to the invention to arotary instead of a translatory displacement.

The graphs of FIGS. 3 to 5 merely show an example. In practice, thedifference between the actual value and the desired value will fluctuatemuch more irregularly. In addition, the transition caused by adisturbance of the derivative is usually not a stepped one, but has acomparatively great directional coefficient.

It is also possible to take a reference value of, for example, 2Rinstead of the reference value R.

It is also possible to multiply the difference by both the first and thesecond derivative, so that a malfunction can be detected even morequickly.

1-5. (canceled)
 6. A method of detecting a malfunction during adisplacement of an element by a drive system, said method comprising thesteps of: determining a difference between a predetermined value and anactual value at regular intervals during the displacement of theelement; determining a derivative of the difference at regularintervals, wherein the difference and the derivative both fluctuatearound an equilibrium value; sampling the values of the difference andthe derivative on one side of the equilibrium value; multiplying thesampled values of the difference and the derivative; comparing themultiplied values to a reference value; and detecting the malfunction ifthe multiplied values are greater than the reference value.
 7. Themethod as claimed in claim 6, wherein the side of the equilibrium valueon which the difference and derivative values are sampled is dependenton the direction in which the element is displaced.
 8. The method asclaimed in claim 6, further comprising the step of: filtering thederivative.
 9. The method as claimed in claim 6, wherein thepredetermined value represents a desired position of the displaceableelement, and wherein the actual value represents an actual position ofthe element.
 10. A component placement device configured to detect amalfunction during a displacement of an element, the component placementdevice comprising: a processor configured to: cause a displacement ofthe element along a path that defines a series predetermined positions;determine an actual position of the element corresponding to each of thepredetermined positions during the displacement of the element,determine a difference between each actual position of the element andthe corresponding predetermined position of the element, determine aderivative of the difference, wherein the difference and the derivativeboth fluctuate around an equilibrium value, sample the values of thedifference and the derivative on one side of the equilibrium value,multiply the sampled values of the difference and the derivative,compare the multiplied values to a reference value, and detect themalfunction if the multiplied values are greater than the referencevalue.
 11. The component placement device according to claim 10, whereinthe processor comprises a control circuit, and wherein the controlcircuit comprises: a drive system configured to displace the elementalong the path that defines the series predetermined positions.
 12. Thecomponent placement device according to claim 11, wherein the controlcircuit further comprises: a controller configured to control the drivesystem.
 13. The component placement device according to claim 12,wherein the control circuit further comprises: an input configured todetermine the actual position of the element corresponding to each ofthe predetermined positions during the displacement of the element. 14.The component placement device according to claim 13, wherein thecontrol circuit further comprises: a difference determinator configuredto determine the difference between each actual position of the elementand the corresponding predetermined position of the element.